Loeb proposed the actress, who plays Gloria Pritchett in the hit TV show, during a vacation to celebrate her 40th birthday in Mexico, reported Ace Showbiz.
"He 'popped the question' while at the Mayan archaeological ruins of Chichen Itza with the entire birthday entourage," a source said.
"She was bouncing around and showing everyone the ring," another source added.
One of Vergara's close friends Fernando Fiore tweeted a picture from the birthday celebration that featured her rocking a supposed engagement ring.
Vergara met Loeb at a Golden Globes party in 2010 and has dated him on and off ever since.
She was previously married to Joe Gonzalez from 1991 to 1993 and has a son, Manolo, with him.